Coal is a major source of energy in the United States. It is increasing in importance because of its abundance within the United States and because of the security and balance of payments problems which arise from reliance upon foreign oil.
Transport problems constitute one of the major difficulties in the use of coal. Many attempts have been made to solve this problem by preparing slurries of coal with carrier liquid and pumping said slurries from one point to another. However, the prior art processes which relate to the pumping of carbonaceous slurries suffer from several distinct disadvantages. In the first place, they generally cannot utilize more than about 50 weight percent of carbonaceous material in the slurry to be pumped; above this critical amount, the slurry ususally becomes unpumpable. In the second place, the slurry must be pumped in a turbulent regime, thus increasing the amount of energy consumed by the process; unless such turbulent regime pumping occurs, the slurry undergoes an unacceptable amount of settling.
3.(a). Prior art teachings regarding the solids content of a coal water slurry and its effect upon pumpability.
The prior art appears to disclose that, in coal-water slurries, when the solids content exceeds a certain critical value, the slurry becomes unpumpable.
An early patent, issued in September of 1920, places this critical value at about 20 weight percent of coal. U.S. Pat. No. 1,390,230 of Bates discloses that "Attempts have been made to carry or force coal through pipes by means of water, but owing to rapid sedimentation it has been possible to convey as a maximum only about 20% by weight of particles under considerable head of water travelling some twenty feet per second. Save under such exceptional circumstances or in rivers, water has not served as a carrier to transport coal. Only very small amounts may be made into a colloidal with water, and so made naturally stable for transportation." (At lines 48-59 of page 1)
In May of 1957, when Clancey et al. issued U.S. Pat. No. 2,791,471, this "critical value" was placed somewhat higher. At lines 13-19 of Column 2 of this patent, it is taught that " . . . coal at the slurry preparation terminal . . . is mixed with water to form a slurry . . . . The resulting slurry should contain about 35 to 55 percent coal by weight." A similar disclosure appears in U.S. Pat. No. 2,791,472 of Barthauer et al., which also issued in May of 1957. At lines 45-49 of the Barthauer et al. patent, it is disclosed that "Coal selected for pipeline shipment is crushed to a suitable size consist, screened and mixed with water to form the slurry for transportation. The resulting slurry should contain about 35 to about 55 percent coal by weight."
In January of 1960, Wasp et al. issued U.S. Pat. No. 2,920,923. In this patent, they discussed the prior art Clancey et al. process and stated that "Certain hydraulic principles relating to pipeline transportation have been set forth in U.S. Pat. No. 2,971,471. A commercial pipeline, embodying these hydraulic principles, has been constructed in Ohio . . . . This coal is mixed with an equal weight of water to comprise a 50 percent aqueous coal slurry." (Lines 24-38 of Column 1)
In January of 1963, U.S. Pat. No. 3,073,652 was issued to Reichl. The Reichl patent appears to disclose that the aforementioned "critical value" of solids content could be as high as 60 weight percent. At lines 30-40 of column 1 it is stated that "The coal particles, that is both the fine and coarse particles, are mixed with water to form a coal-water slurry having a solids concentration of between 35 and 60 percent by weight coal particles. It has been discovered that a slurry prepared as described above is dynamically stable in that the tendency of the larger sized coal particles to settle out of the slurry is reduced . . . . " However, as is taught in the Cole et al. patents, the coal concentrations taught in Reichl appear to be calculated on a "wet basis" and, thus, apparently correspond to "dry basis" coal concentrations of up to about 45 weight percent.
In February of 1965, U.S. Pat. No. 3,168,350 was issued to Phinney et al. In the Phinney et. al. patent, reference is again made to the prior art Clancey et al. process disclosed in U.S. Pat. No. 2,791,471. With regard to the prior art process, Phinney et al. stated that "The process employed to transport the coal as an aqueous slurry through this commercial pipeline is set forth in U.S. Pat. No. 2,791,471 . . . . The coal particles having the above size distribution and nominal top size are mixed with water to prepare a slurry comprising 35-55 percent by weight of the coal particles and the remainder water." (Lines 25-37 of column 1)
In December of 1976, a U.S. patent issued which disclosed that, at above a solids content of 50 percent of coal (dry basis), a slurry is unpumpable. U.S. Pat. No. 3,996,026 of Cole disclosed that "Ordinarily, a pumpable slurry of solid fuel or coal requires the addition of water to the powdered fuel to form a slurry containing not more than about from 40 to 45 wt. % coal. As the solids content increases above this range the slurry becomes increasingly difficult to pump and at about 50% solids content, it is unpumpable (at lines 29-36 of column 1). Cole also teaches that the as-mined coal contains a substantial amount of moisture and, unless it is dried, a slurry containing 50 weight percent of such as-mined coal in fact contains substantially less than 50 weight percent of coal. At lines 37-47 of his patent, he discloses that "Actually such slurries contain in excess of 50% water as there is a considerable amount of water in coal as mined . . . . The coal or solid fuel also contains chemically bound water . . . depending on the type of solid fuel, a pumpable slurry may contain as little as 30 to 35 wt. % solids on a dry basis."
In May of 1978 yet another patent issued disclosing that a pumpable coal-water slurry could contain no more than about 40 to about 45 weight percent of coal. U.S. Pat. No. 4,088,453 disclosed that "The amount of water necessary to form a pumpable slurry depends on the surface characteristics of the solid fuel . . . in the case of a slurry made up of solid fuel particles most of which will pass through a 200 mesh sieve it has been found that ordinarily, a pumpable slurry must contain from 55 to 60 wt. % water." (Column 1, lines 25-46)
In August of 1978, U.S. Pat. No. 4,094,035 issued to Cole et. al. It also contained disclosure that a coal-water slurry with more than 50 weight percent of coal was unpumpable; the portion of U.S. Pat. No. 3,966,026 quoted hereinabove was included verbatim in the Cole et. al. U.S. Pat. No. 4,104,035 patent.
In summary, the prior art appears to indicate that the coal content of coal-water slurries generally must be maintained at less than about 50 weight percent in order for these slurries to be pumpable.
3.(b). Prior art teachings regarding the solids content of a coal-water slurry and its effect upon stability.
The prior art appears to disclose that the use of a high solids content in a coal-water slurry has an adverse affect upon the stability of the slurry. U.S. Pat. No. 4,242,098 discloses that "Aqueous slurries of finely ground coal containing more than about 55 weight percent solids are difficult to transport using conventional slurry pumps because as the solids level is increased above about 50 weight percent the water and solids tend to separate causing coal particles to build up in various areas throughout the pumping system." (Column 1, lines 15-21)
3.(c). Prior art teachings regarding the solids content of a coal-oil slurry and its effect upon pumpability.
U.S. Pat. No. 3,907,134 teaches that the use of more than 50 weight percent of coal in a coal-oil mixture has an adverse effect upon the pumpability of the mixture. At lines 41-50 of Column 1, it is disclosed that "The fuel oil and particulate carbonaceous material are preferably mixed in metered amounts . . . . For most users about 5 weight percent of coal or less is not normally economically interesting, and above 50 weight percent of pulverized coal begins to cause undesirable flow characteristics in the slurry."
U.S. Pat. No. 3,846,087 discloses that, with regard to carbon-oil slurries, " . . . major problems are encountered in maintaining the carbon-oil slurry pumpable when the carbon content thereof exceeds about 4 weight percent in naphtha, gas oil, lube oil, shale oil, decanted oil, gasoline, crudes deficient in +1000.degree. F. boiling material or in hydrogen deficient in +1000.degree. F. boiling material. Above this figure, the slurry does not flow and upon heating only becomes more gel-like" (lines 8-15 of Column 1).
3.(d). Prior art teachings about pumping a coal-liquid slurry in a turbulent regime.
The prior art appears to disclose that most coal-liquid slurries must be pumped in a turbulent flow regime. "Turbulent flow" has been defined as "a. Fluid motion in which random motions of parts of the fluid are superimposed upon a simple pattern of flow . . . . b. A fluid flow in which there is an unsteady motion of the particles, the motion at a fixed point being inconstant. Turbulent flow occurs at a speed above the critical velocity of Reynolds." A dictionary of mining, mineral, and related terms, edited by Thrush (U.S. Department of the Interior, 1968), page 1175. The "Reynolds' critical velocity" " . . . is that at which the flow changes from laminar to turbulent, and where friction ceases to be proportional to the first power of the velocity and becomes proportional to a higher power--practically the square." A dictionary of mining, mineral, and related terms, supra, page 279. It appears that the prior art teaches that, in order to pump a coal-fluid slurry, it must be pumped at a speed above the critical velocity of Reynolds.
U.S. Pat. No. 2,920,923 of Wasp et al. discloses that there is a "minimum non settling velocity" which is required to prevent settling of solids in a moving slurry. This minimum settling velocity is required because " . . . the transportation of slurries is made possible by the existence of turbulent eddy currents which continually resuspend the coarser particles of the slurry." (Column 4, lines 27-30)
U.S. Pat. No. 3,206,256 of Scott discloses that "The transportation of such granular material is known and presents the problem of maintaining the granules in suspension within the carrier liquid. Thus, when as is usually the case, the particles have a density greater than that of the carrier liquid they tend to settle to the bottom of the pipe line unless the flow velocity is sufficient to create the turbulence required to counteract settling . . . " (Column 1, lines 22-28).
U.S. Pat. No. 3,268,263 of Van Olphen discloses that " . . . the transportation of granular materials . . . in a carrier liquid through a pipe line is known. Such transportation presents the problem of maintaining the granules in suspension within the carrier liquid due to the different, usually greater density of the granules. The settling problems, considerations affecting settling rates, and certain remedies . . . are considered in the U.S. Pat. Nos. 2,929,923 to Wasp et al. and 2,610,900 to Cross. As is evident from these disclosures, the granules are kept in suspension within the carrier liquid through turbulence, and for each system there is a `minimum non-settling velocity,` which must be maintained if settling is to be avoided." (Column 1, lines 21-36)
U.S. Pat. No. 3,514,217 of Reiss discloses that "When transporting solids as a slurry in a pipeline, the flow velocity of the slurry must be maintained above some critical value, V.sub.c, in order to fully suspend the solid. If the flow velocity falls below the critical velocity, solids will settle on the bottom of the pipe. This is known as unstable slurry flow and may result in the pipeline becoming plugged . . . " (Column 1, lines 28-34)
It is the object of this invention to provide a process for pumping a high-solids content (60-82 volume percent) carbonaceous slurry in the laminar regime.